[ABNF] Update some documentation.

This does not change the grammar. It merely updates some text in the comments,
which shows a rule that was changed at some point but its copy in the comments
was not properly updated.

Thanks to @bendyarm for noticing this.

docs/grammar/README.md

@@ -328,7 +328,7 @@ and therefore we have rules like
 
 
 ```
-unary-expression = primary-expression
+unary-expression = postfix-expression
                  / "!" unary-expression
                  / "-" unary-expression
 ```

docs/grammar/abnf-grammar.txt

@@ -263,7 +263,7 @@
 ; the relative precedence and the associativity of expression operators,
 ; and therefore we have rules like
 ;
-;  unary-expression = primary-expression
+;  unary-expression = postfix-expression
 ;                   / "!" unary-expression
 ;                   / "-" unary-expression
 ;